1. Field of the Invention
This invention relates to the field of detecting parameters such as the pressure or temperature inside a tyre, and more specifically to variations enabling the detection unit to be attached to the wheel rim under optimal conditions.
2. Related Art of the Invention
There are several solutions in the prior art for attaching a detection unit. These solutions include the one adopted by the invention, which involves attaching, inside the rim, the detection unit which contains sensors corresponding to the parameters to be monitored, to the tyre inflation valve body.
The applicant has noted that the implementations of this attachment solution known in the prior art have certain disadvantages, as described below.
American documents U.S. Pat. No. 6,055,855 and 5,844,131 propose attaching a detection unit inside a tyre by means of a valve body, which involves an attachment assembly in which said body is preformed at a first end to pivotably receive the unit and at its second end to enable a fixing nut to be screwed. The valve body is also preformed on its internal portion, with a shoulder on which an elastomer seal is arranged so that the screwing action of the nut coming into contact with the external surface of the rim enables the shoulder to move toward the internal surface of the rim, thus fixing the elastomer seal against said surface. The elastomer seal is preformed to penetrate the aperture made in the rim to enable the attachment of the valve and to be inserted in this aperture between the valve body and the rim. The screwing action also ensures that said unit is urged against the internal surface of the valve.
This embodiment has several disadvantages noted by the applicant, including the following:                When the wheel rotates at high speed, the valve body is subjected to very high stresses which are transferred to the seal placed between the body and the rim. However, the lifetime of an elastomer sealing element which is not designed to withstand such stress is considerably reduced, leading to pressure loss and failure.        The seal is urged against the internal surface of the rim, which in most cases has a surface planarity capable of damaging the seal and/or reducing its sealing ability.        As the seal is an integral part of the attachment assembly, the screwing force must be adjusted and not exceed a threshold beyond which the seal would be damaged. However, this controlled screwing is not always respected.        Moreover, insofar as the screwing force is performed as prescribed, the presence of the fixing nut at the exterior makes unintentional unscrewing very easy.        As it is simultaneously subjected to the screwing pressure, tangential forces due to the rotation of the wheel, and the pressure and temperature of the tyre, the seal is subjected to a phenomenon of creep, in which its internal portion is dilated and the part inserted between the cylindrical surface of the valve and the cylindrical surface of the aperture made in the rim for receiving said valve is compressed.        By urging the unit against the rim, the latter is subjected to the temperature of the rim.        Such an assembly is suitable for an attachment to an automobile wheel rim in which the valve is not present in the plane of symmetry of the rim.        
A modification is described in the French document FR2834245 which, on the basis of the same attachment principle, has the advantage of proposing a valve body with an internal shoulder against which the elastomer seal is positioned, with the latter being preformed so as to form a mechanical stop opposing the screwing force of the external nut so that the screwing force exerted on the seal does not exceed the threshold value, beyond which it will be damaged. Although this assembly proposes a solution to one of the problems of the prior art, it retains the other disadvantages described above.
In addition, another disadvantage found in the prior art relates to the unit itself which, for reasons of sealing and holding the components that it receives in position, is conventionally filled with a plastic material that increases the weight of the unit and makes it impossible to recycle it. The speeds and accelerations to which the unit is subjected make any additional mass become an additional stress transferred to the assembly for attaching the unit to the valve and consequently to the seal in the solutions of the prior art.